1. Field of the Invention
The present invention relates to a light-wave distance measuring apparatus used or for measuring a distance to a target object.
2. Description of the Related
FIG. 5 is an electric circuit block diagram of a prior art light-wave distance measuring apparatus, i.e., for apparatus electrically measuring a distance up to a target object using light waves.
This prior art apparatus comprises a light intensity modulator 1, a reference oscillator 2, a local oscillator 3, a first mixer 4, a demodulator 5, a second mixer 6, a phase meter 7 and a CPU 8. The reference oscillator 2 produces a reference signal. The light intensity modulator 1 produces a light intensity modulated signal (outgoing beam of light) L-1 from the reference signal from the reference oscillator 2 and transmits it to a target object (the distance to which to be measured).
The demodulator 5 demodulates a reflected light L-2 received from the target object. The first mixer 4 mixes the reference signal from the reference oscillator 2 with the local oscillator signal from the local oscillated from local the frequency of the reference signal. The second mixer 6 mixes the demodulated signal from the demodulator 5 with the local oscillator signal for converting the frequency of the demodulated signal. The phase meter 7 detects a phase difference between the frequency-converted signal from the first mixer 4 and the frequency-converted signal from the second mixer 6. The CPU 8 receives and processes a signal from the phase meter 7 indicative of the detected difference and determines the distance to the target object.
Since it is supposed that the prior art light-wave distance measuring apparatus is intended to be used by itself the apparatus usually has no means for overcoming or preventing an interference caused by a light wave transmitted from another light-wave distance measuring apparatus. Accordingly, when other light-wave distance measuring apparatus are mounted on vehicles which gather in a small area, they are respectively exposed to interference created by a light waves from, for example, a vehicle. This causes a problem in distance measurement.
In addition, it is supposed that almost all of prior art light-wave distance measuring apparatuses are employed in a case where both the distance measuring apparatus and the target object are static. Therefore, even if the prior art apparatus can measure a distance to the target object, there is a problem that the prior art apparatus fails to provide sufficient data for distance measurement when both the vehicle on which the apparatus is mounted and the target object move away from or to each other, such as when a vehicle in front of the measuring apparatus moves essentially at the same speed in the same direction as the vehicle on which the apparatus is mounted. The safety factor in the latter case is much lower than when both the apparatus and the target object are static. These skilled in the art have been seeking to solve